1. Field of the Invention
The present invention relates to a silica fine particle, toner, two-component developer and image forming method using the toner all of which can be suitably used to form and develop an electric latent image in electrophotography, static printing or toner jet method.
2. Description of the Related Art
A large number of methods have conventionally been known as electrophotography. In general, an electrostatically charged latent image is formed on a photosensitive member using a photoconductive substance by various units, the latent image is developed with a toner to form a toner image, and the toner image is transferred to a transfer material such as paper as required and is fixed on the transfer material by heat, pressure, both of them, or the like to obtain a copy or printed matter. Residual toner particles remaining on the photosensitive member without being transferred to the transfer material are removed from the photosensitive member by a cleaning step.
For the step of cleaning the photosensitive member, methods such as blade cleaning, fur brush cleaning, or roller cleaning have been employed. The above method is used to scrape off or check the transfer residual toner on the photosensitive member mechanically to collect the residual toner in a waste toner container. However, since a member used for cleaning is pressed against the surface of the photosensitive member, the surface of the photosensitive member is apt to wear away.
Since the apparatus is provided with a cleaning unit, the apparatus becomes bulky inevitably, which is an obstacle to the downsizing of the apparatus.
Further, a system which is free from the production of waste toner is desired from an ecological point of view.
For example, JP 5-69427 B proposes an image forming apparatus which employs a technology so called “developing/cleaning” or “cleaner-less”. In the image forming apparatus, one image is formed each time the photosensitive member makes one revolution in order to prevent the transfer residual toner from affecting the same image. JP 64-20587 A, JP 2-259784 A, JP 4-50886 A and JP 5-165378 A propose that the transfer residual toner is scattered over a photosensitive member by a scattering member to prevent the formation of a pattern so that the transfer residual toner is hardly seen on an image even when the same surface of the photosensitive member is used several times for one image.
However, when a cleaner-less system is attained by preventing the formation of a pattern with the residual toner, a device for applying a voltage to a member for preventing the formation of a pattern is required, thereby making it difficult to downsize the whole apparatus.
JP 2-51168 A proposes to obtain stable charging properties by using a spherical toner or spherical carrier in a cleaner-less electrophotographic printing method. However, the publication is utterly silent about physical properties of the toner, particularly an external additive existent on the surface of the toner, which are the major factors of the fusion of the toner to a member or drum, or the like.
JP 5-2287 A proposes to obtain stable charging properties by setting the resistance and the amount of charge of a toner to appropriate values in a cleaner-less electrophotographic printing method. However, the publication is utterly silent about the physical properties of the toner, particularly an external additive existent on the surface of the toner.
Further, JP 6-250566 A, JP 8-292640 A, JP 11-38730 A and JP 11-311890 A propose various cleaner-less electrophotographic printing systems but the publications are utterly silent about a preferred external additive suitable for use in the cleaner-less systems.
As demand for obtaining high quality image from users is strong, the physical properties of a toner, particularly the form of an external additive existent on the surface of the toner are important from this point of view. JP 11-174731 A or the like proposes a toner having excellent development stability by specifying a particle diameter and form of an external additive. However, this application does not refer to a coarse particle contained in the silica fine particle used as the external additive. When the silica fine particle contains a large amount of the coarse particle, the problem of contamination of a roller or an image bearing member in the image forming comprising a charging roller as a charging member. Therefore, further improvement is required.
Various organic photoconductive materials have recently been developed as a photoconductive material for use in an electrophotographic photosensitive member. In particular, a function separation type organic photoconductive material including a charge generation layer and a charge transport layer has been implemented and used in copiers, printers, facsimiles, and the like. As a charging unit for those electrophotographic apparatuses, units making use of corona discharge have been used. However, when corona discharge is used, ozone is generated.
As technology for solving the above problem, there has been developed a charging method in which a charging member such as a roller or blade is contacted to the surface of a photosensitive member to form a narrow space in the vicinity of its contact portion in order to form discharge which can be interpreted by so-called “Paschen's law”, thereby suppressing the generation of ozone as much as possible. Out of those, a roller charging method using a charging roller as the charging member is preferably used from the viewpoint of charging stability.
Since this charging is carried out by discharging from the charging member to a charged body, charging is started by applying a voltage equal to or higher than a certain threshold voltage. For example, when a charging roller is contacted to a photosensitive member having an about 25 μm-thick photosensitive layer and containing an organic photoconductive material, the surface potential of the photosensitive member starts to rise at a voltage equal to or higher than about 640 V and then increases linearly at an inclination of 1 based on an application voltage. This threshold voltage is hereinafter defined as a charge start voltage Vth. That is, to obtain the surface potential Vd of the photosensitive member, Vd+Vth, that is, a DC voltage equal to or higher than that required must be applied to the charging roller. Further, as the resistance value of the charging roller is changed by environmental variations, it has been difficult to control the potential of the photosensitive member to a desired value.
Therefore, in order to uniformize charge, as disclosed by JP 63-149669 A, for example, a DC+AC charging system is employed in which a voltage obtained by superimposing an AC voltage having an inter-peak value of 2×Vth or higher on a DC voltage equivalent to desired Vd is applied to a contact charging roller. This is aimed to achieve the effect of uniformizing potential by AC. The potential of a member to be charged converges to Vd at the center of the peak of the AC voltage and is hardly affected by an external disturbance such as environmental variations.
However, since an essential charging mechanism of this charging system makes use of a discharge phenomenon from a charging member to a photosensitive member, a voltage required for charging must be equal to or higher than the surface potential of the photosensitive member as described above. Further, the generation of vibration and noise (to be referred to as “AC charging sound” hereinafter) in the charging member and the photosensitive member caused by the electric field of an AC voltage and the deterioration of the surface of the photosensitive member by discharge become marked as new problems to be solved.
JP 5-19662 A proposes the use of a secondary particle obtained by fusing a primary polymer particle in a toner, JP 4-296766 A proposes the use of a polymerization toner which transmits exposure light for a photosensitive member, and JP 5-188637 A proposes the use of a toner having a specific volume average particle diameter, number average particle diameter, amount of charge, area ratio of its projection image, BET specific surface area, and the like. However, an excellent image forming method using a development/cleaning system is still desired.
When a “development/cleaning system” or “cleaner-less” technology is used, the transfer residual toner cuts off exposure light and interferes with the formation of an electrostatically charged latent image, thereby making it impossible to control the potential to a desired value, with the result of the formation of a negative memory on an image. Further, when there is a large amount of the transfer residual toner, the residual toner cannot be entirely collected in the development step and a positive memory is apt to be formed on the image. Even when a member for preventing the formation of a pattern is used, image quality tends to lower.
As for toner, it is generally known that inorganic fine particles having a small particle diameter are added externally to coloring particles (toner particles) in order to obtain excellent developability, cleanability and transferability by adjusting the charging properties, fluidity, or the like of the toner.
However, it has been confirmed that when the toner containing the above externally added inorganic fine particles each having a small particle diameter is used for a long time, the inorganic fine particles each having a small particle diameter are buried in the surface of the toner by stress from a carrier when the toner is used as a two-component developer, stress from a developer coating blade or developer supply roller when the toner is used as a one-component developer, or collision with an inner wall of a developing device or stirring blade, or between toners.
To prevent the inorganic fine particles each having a small particle diameter from being buried, a method making use of inorganic fine particles each having a large particle diameter is effective as disclosed by JP 4-204751 A, JP 5-346682 A, JP 6-313980 A, JP 6-332235 A and JP 7-92724 A.
The addition of inorganic fine particles each having a large particle diameter provides a so-called “spacer effect”, so that direct contact between the surface of the toner having the inorganic fine particles each having a small particle diameter adhered thereto and a carrier, developer coating blade, developer supply roller, the inner wall of a developing device, stirring blade and another toner is prevented, thereby reducing stress. Therefore, the inorganic fine particles each having a small particle diameter are prevented from being buried and the service life of the toner is extended.
Further, to maintain this spacer effect, silica is preferably used as the inorganic fine particles each having a large particle diameter. The reason for this is as follows. The inorganic fine particles each having a large particle diameter have relatively lower electrostatic adhesion to the surface of a toner than that of the inorganic fine particles each having a small particle diameter. Therefore, the inorganic fine particles each having a large particle diameter are apt to be separated from the surface of the toner, consumed by development or the like and reduced in number, whereby its spacer effect tends not to last for a long time. Since silica has a larger amount of charge and higher adhesion to the surface of a toner than other inorganic materials, when silica is used as the inorganic fine particles each having a large particle diameter, separation can be suppressed and the spacer effect can be maintained.
However, a toner to which inorganic fine particles each having a small particle diameter and silica having a large particle diameter are externally added is apt to cause a so-called “charge-up” that charging properties become too high in a low-humidity environment. Thus, the toner has poor environmental stability.
Meanwhile, JP 7-104501 A proposes a toner which contains hydrophobic silica having a particle diameter of 15 to 20 nm, hydrophobic silica having a particle diameter of 13 nm or less, and alumina as external additives. Although this toner achieves excellent environmental characteristics in a two-component developer containing a carrier, the hydrophobic silica having a particle diameter of 15 to 20 nm separates from the surface of the toner in a non-magnetic one-component developer, thereby making it impossible to obtain a satisfactory spacer effect and promoting the silica to be buried in the surface of the toner. As a result, an increase in fogging, the occurrence of a cleaning failure and a reduction in transfer efficiency are seen. Also satisfactory environmental characteristics are not obtained, and a reduction in the density of an image caused by the charge-up and image nonuniformity are observed. Supposedly, this is because a mechanical stress from a blade which is a charging member is larger in non-magnetic one-component development than in two-component development.
The cleaning and preventing fusion of the photosensitive member are being improved by the physical properties of an external additive. The BET specific surface area of an inorganic fine particle has often been used as an index for the particle diameter of the inorganic fine particle. Although this method roughly indicates the size of a particle, it is difficult to know the size of a primary particle and the particle diameter of a particle of higher order, which is an agglomerate of primary particles, even when their specific surface areas are the same.
JP 7-319201 A and JP 11-167250 A also propose technology for adding silica having a specific particle diameter distribution to a toner externally. In those publications, the particle diameter distribution of the inorganic fine particles is measured by an aperture type particle diameter distribution meter. In this measurement method, the measurement range is so narrow that particles larger than the aperture diameter cannot be measured and particles each having a small particle diameter of about 1 μm or less cannot be measured owing to detection limits. Therefore, it is difficult to know the particle diameter, and further improvement in the control of a particle diameter distribution is required. It is noted that those publications are mainly aimed to control the amount of coarse particles.
In view of this, the development of a method for measuring the size of a particle and the degree of agglomeration and novel inorganic fine particles based on that measurement method is desired. JP 10-67510 A proposes silica having a particular specific surface area and discloses the particle diameter distribution of the silica but is utterly silent about the particle diameter distribution of fine particles each having a particle diameter of less than 1 μm.